1. Field of the Disclosure
The present invention is generally related to image sensors, and more specifically, the present invention is directed to high dynamic range image sensors.
2. Background
An image capture device includes an image sensor and an imaging lens. The imaging lens focuses light onto the image sensor to form an image, and the image sensor converts the light into electric signals. The electric signals are output from the image capture device to other components of a host electronic system. The electronic system may be, for example, a mobile phone, a computer, a digital camera or a medical device.
The demands on the image sensor to perform over a large range of lighting conditions, varying from low light conditions to bright light conditions are becoming more difficult to achieve as pixel cells become smaller. This performance capability is generally referred to as having high dynamic range imaging (HDRI or alternatively just HDR). In conventional image capture devices, pixel cells require multiple successive exposures to achieve HDR.
FIG. 1 is a circuit diagram showing a four-transistor (“4T”) pixel cell 100. As shown, pixel cell 100 includes photosensitive element 110, transfer transistor 120, reset transistor 130, floating diffusion (“FD”) 180, source follower (“SF”) transistor 140, row select transistor 150, dual conversion gain transistor 160 and capacitor 165.
During operation of pixel cell 100, transfer transistor 120 receives a transfer signal TX, which transfers charge accumulated in photosensitive element 110 to floating diffusion FD 180. Reset transistor 130 is coupled between power supply VDD and floating diffusion FD 180 to reset the pixel cell (e.g., to discharge or charge floating diffusion FD 180 and/or photosensitive element 110 to a preset voltage) under control of reset signal RST. FD 180 is also coupled to control the gate of SF transistor 140. SF transistor 140 is coupled between power supply VDD and row select transistor 150. SF transistor 140 operates as a source follower providing a high impedance connection to floating diffusion FD 180. Under control of a select signal SEL, row select transistor 150 selectively provides an output of the pixel cell to a readout column line, or bit line 170.
Capacitor 165 and dual conversion gain transistor 160 are coupled in series between power supply VDD and floating diffusion FD 180, with dual conversion gain transistor 160 coupled to FD 180 and capacitor 165 coupled to power supply VDD. The capacitance of capacitor 165 may be added to FD 180 by asserting dual conversion gain signal, DCG, thereby decreasing the conversion gain of the pixel cell 100.
Photosensitive element 110 and FD 180 are reset by temporarily asserting the reset signal RST and the transfer signal TX. An image accumulation window (e.g., an exposure period) is commenced by de-asserting the transfer signal TX and permitting incident light to photogenerate electrons in photosensitive element 110. As photogenerated electrons accumulate in photosensitive element 110, the voltage on photosensitive element 110 decreases. The voltage or charge on photosensitive element 110 is indicative of the intensity of the light incident on photosensitive element 110 during the exposure period. At the end of the exposure period, the reset signal RST is de-asserted to isolate FD 180 and the transfer signal TX is asserted to allow an exchange of charge between photosensitive element 110 and FD 180, and hence the gate of SF transistor 140. The charge transfer causes the voltage of FD 180 to change by an amount that is proportional to photogenerated electrons accumulated on photosensitive element 110 during the exposure period. This second voltage biases SF transistor 140, which in combination with the select signal SEL being asserted, drives a signal from row select transistor 150 to the bit line 170. Data is then readout from the pixel cell 100 through bit line 170 as an analog signal.
By changing the conversion gain of the pixel cell 100 between successive image captures, the HDR of the resultant image can be increased. However, this would increase amount of time required to capture and readout one HDR image and affect the performance of the image capture device.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.